Litz Wire in Induction Heaters: Where It Sits Inside the System and How It Is Calculated

Let us break the system down node by node, going from the wall socket to the workpiece. At each node we show what sits there, why this particular construction is used, how the wire is calculated, and which DIADA catalog series fits. The fundamentals of skin effect and proximity effect are covered in the article on skin effect, and Sullivan's optimum is covered in the LLC article. Here we focus on the induction-heating specifics of the system as a whole.

Key terms

The system power chain: what sits where

An industrial induction heater is a cabinet or a monoblock with five main nodes arranged in series. Energy flows through them from the three-phase 380 V / 50 Hz mains to the induction coil with the workpiece.

Of the five nodes, litz wire sits in three: the PFC inductor at the input, the RF cable between the inverter and the matching transformer, and the windings of the matching transformer itself. In the fourth node (the coil) litz wire is used selectively. In the first one (the Graetz bridge + DC bus) there is no litz wire at all.

In total, a 30 kW system uses: PFC inductor 3–5 kg, RF cable 0.5–2 kg, matching transformer 4–8 kg, coil 1–3 kg. That is 8.5–18 kg of litz wire. Of this, about 75 % is the matching transformer and the PFC inductor. By weight, the coil is the smallest item.

Node 1: the input PFC inductor

A power factor corrector (PFC) at the input is needed for two reasons: the IEC 61000-3-2 standard (since 2001 it has required cos φ ≥ 0.95 for equipment >600 W) and a technical one (without PFC the DC bus capacitance has to be increased several-fold).

The topology is usually a boost converter: an inductor, a switch (IGBT or SiC MOSFET), a diode, and an output capacitor. The PFC operating frequency is 50–100 kHz for IGBT and 100–250 kHz for SiC.

In continuous conduction mode, the PFC inductor carries an almost sinusoidal current at the 50 Hz mains frequency with an HF ripple from the pulse-width modulation (PWM) of the switch at 20–30 % amplitude. The litz wire is calculated for the HF component. At 100 kHz and 100 °C, the skin depth in copper is 0.21 mm. The 0.15 mm or 0.2 mm series applies.

For a 30 kW PFC inductor (I_RMS = 50 A):

• Cu cross-section: 12.5 mm² at a current density of 4 A/mm² (air cooling)
• Strand count for the 0.15 mm series: 707 → catalog 705×0.15 mm, 0.15 mm series
• Coating: PET film (Litz PET), breakdown voltage 1000 V
• F_R at 100 kHz with a two-layer winding: 1.15–1.25

In single-phase systems up to 5 kW, the PFC inductor is usually wound with PETV-2 wire without transposition. Litz wire appears in the PFC starting at a power of 8–10 kW.

Node 2: the inverter and resonant capacitor

The inverter itself contains no litz wire. Semiconductor switches sit on a water-cooled heat sink, ceramic DC-link capacitors, bus bars made of 3–5 mm copper plates. The connections are short, and parasitic inductance is more critical than losses.

The resonant capacitor sits next to the inverter and closes the resonant tank. In industrial 30–500 kW systems it is a bank of parallel ceramic KEMET or Murata PHE850-series capacitors at 2–6 kV, or WIMA FKP1 film capacitors.

Between the capacitor and the matching transformer is the RF bus bar. Over a short distance — a copper plate. For several meters to the remote head — a coaxial litz cable (see below).

Node 3: RF buses and the coaxial litz cable

Transmitting power at RF is a non-trivial task. At 200 kHz, 200 A, and 1000 V you cannot run ordinary wire: skin effect, EM interference, inductive losses. Over 2 m the losses can amount to 500–1500 W (2–5 % of the system power).

Construction of the coaxial litz cable

The standard is the coaxial litz cable (patent US 20020011913, Inductotherm 2002):

• Inner conductor — litz wire of the 0.1 mm series (e.g. 500×0.1 mm)
• Insulation — PTFE tape or a PEEK jacket rated 1500–3000 V
• Outer conductor — litz wire placed concentrically around the insulation
• Outer jacket — a PTFE hose or corrugated polyamide
• Water cooling channel — inside the hollow conductor or alongside

The coaxial construction reduces parasitic inductance by a factor of 5–10: the inner and outer conductors carry antiparallel currents. EM radiation to the outside is almost zero.

Calculating litz wire for a coaxial RF cable

For a typical 30 kW cable at 250 kHz: current 200 A, length 2 m, cross-section of each conductor 5 mm² at a current density of 40 A/mm² (water cooling). The 0.1 mm series, strand count 637 → catalog 630×0.1 mm. Coating — PTFE film (Litz 200 F), breakdown voltage 3000 V.

The DIADA catalog includes the Litz 200 F and Litz 200 fPE grades. We have experience building coaxial RF cables. We do not yet produce a finished cable with an integrated water channel in series — this is a promising line for joint development. More details in the "Water-cooled litz cable" section.

When a coaxial cable is needed

Node 4: the matching transformer — the main litz-wire consumer

The matching transformer is the largest litz-wire item in the system. It converts the inverter's RF voltage (400–800 V) into the coil voltage (40–200 V) and steps up the current. A typical turns ratio is 1:4 to 1:10.

Why this node is the most "litz-wire-hungry"

Unlike the coil, where there is a choice between a tube and litz wire, in the matching transformer there is no choice — only litz wire. In a transformer the windings are packed tightly, in multiple layers, surrounded by a core with a strong magnetic field. This is the regime of maximum proximity effect. A copper tube would have an F_R of 8–15 at 200 kHz. That turns the transformer into a heater of itself. Every manufacturer — Inductotherm, Ambrell, EFD, Inductoheat — builds matching transformers with litz wire only.

Construction and calculation

The core is Ferroxcube 3C95 ferrite or FT-3KL nanocrystalline material. The geometry is a toroid or ETD/PQ. For a 30 kW transformer at 250 kHz: ETD 59/31/22. Winding interleaving P-S-P is mandatory.

F_R at 250 kHz with P-S-P: 1.30–1.40. For transformers above 100 kW the secondary is rectangular litz wire (LitzPr), with a fill factor of 85–90 % versus 70 % for round wire.

Insulation

At voltages of 400–800 V between windings, the inter-winding insulation must withstand 3000 V. The standard: litz wire with PTFE film (Litz 200 F), an inter-winding PTFE tape or epoxy impregnation, thermal class 200 °C.

Cores for the RF transformer: ferrite, nanocrystalline, FINEMET

At frequencies of 100–500 kHz electrical steel does not work — losses grow as f². One of three classes is used: ferrites, nanocrystalline alloys, amorphous alloys.

Mn-Zn ferrites

The standard solution of the last 40 years. Manufacturers: Ferroxcube, Hitachi Metals (Magnetics Inc.), TDK, Acme Electronics.

Pros: low specific losses, availability, low price (an ETD 59/31/22 in 3C95 costs 800–1200 ₽). Cons: low saturation flux density (0.33 T), brittleness.

Nanocrystalline alloys: VITROPERM, FINEMET, NANOPERM

A ribbon of Fe-Si-B-Cu-Nb alloy (14–20 µm) heat-treated to form nanoscale grains in an amorphous matrix. Composition: Fe73.5 Si13.5 B9 Cu1 Nb3.

Manufacturers: VACUUMSCHMELZE (Germany) — VITROPERM 500F, 550HF (since April 2025); Hitachi Metals/Proterial (Japan) — FINEMET FT-3KL, FT-3M; Magnetec (Germany) — NANOPERM M-038, M-067.

In the work by Dilks et al. (M. Dilks et al., IEEE PCIM 2022), VITROPERM 550HF has specific losses about 30 % lower than 500F across a wide range of flux densities.

The specific losses of nanocrystalline material are higher than ferrite at the same flux density. But the operating flux density is 3–4 times higher: 1.0 T versus 0.3 T. A nanocrystalline core for the same power is 1.75 times smaller in volume (shown in the work by Petrescu et al., Materials 2021, vol. 14, no. 7745). Critical for EV charging stations and embedded equipment.

Metglas amorphous alloys

An intermediate option. The manufacturer is Metglas Inc. (USA, Conway), with the 2605SA1 and 2714A series. In induction heating it is used in a limited way, more often in the filters of low-frequency systems (forging heating up to 30 kHz).

Choosing a core for the task

• f up to 100 kHz, up to 30 kW — ferrite N87 or 3C90.
• f 100–500 kHz, 30–300 kW — ferrite 3C95 or N97.
• f 500 kHz – 1 MHz, up to 30 kW — ferrite 3F4 or nanocrystalline VITROPERM 500F.
• f above 1 MHz — ferrite 3F4 or nanocrystalline VITROPERM 550HF.
• EV charging stations, high power density — VITROPERM 550HF, no alternative. A mass saving of 30–40 %.
• Military and aerospace equipment — nanocrystalline (−40 to 200 °C).

Node 5: the induction coil — copper tube or litz wire

This is the work coil that wraps around the workpiece. It is outside the cabinet, sometimes next to a workpiece glowing at 1200 °C.

When the coil is made of tubing

Most coils are water-cooled copper tubes of 6×8 mm, 8×12 mm, 10×15 mm cross-section. Ambrell, EFD, Inductoheat, and VNIITVCh use this by default. Reasons: below 100 kHz F_R = 1.1–1.3, direct integration of water cooling, rigidity, ease of non-standard geometries, resistance to glowing-hot workpieces.

When the coil is made of litz wire

The main reason is high frequency. At 500 kHz+ the skin depth in the wall of a 1 mm tube is less than 90 µm. F_R reaches 8–15. Litz wire of the 0.1 mm series gives 1.3–1.5.

Beyond that — domestic cooktops (a flat spiral coil), precision hardening with a complex shape, EV wireless charging (a flat litz coil is the standard per SAE J2954).

Calculating litz wire for the coil

The coil works without a core. The magnetic field closes through the workpiece and air. The calculation goes through 3D FEMM or Ansys Maxwell with modeling of the workpiece.

For a typical 30 kW coil at 250 kHz: current 170 A, Cu cross-section 2.1 mm² at a current density of 80 A/mm², the 0.1 mm series, catalog Litz 200 fPE 255×0.1 mm, F_R = 1.28.

Resonant topologies and the soft-switching regime

Resonance is needed because of the reactive nature of the load (cos φ 0.1–0.3) and to provide soft switching (ZVS/ZCS).

Half-bridge inverter with series resonance

The standard for low-power 1–30 kW units. Used in domestic cooktops and hardening units up to 30 kW. Wolfspeed CAS300M12BM2 or Infineon FF300R12KE7.

Full-bridge inverter

The standard for industrial 30–500 kW units: Inductotherm VIP, EFD Sinac, Ambrell EKOHEAT, VNIITVCh RF generators. Control — phase-shift PWM.

Class E (single-transistor)

Efficiency up to 98 % at the optimal load. Used in MHz inverters for jewelry and brazing of medical needles. A typical set: GaN Systems GS66516B, an inductor of 0.071 mm litz wire on a VITROPERM nanocrystalline core.

Single-stage AC-AC resonant on SiC

Described in the work by Sasaki et al. (S. Sasaki et al., IEEE IECON 2018). Direct conversion of 50 Hz into 100 kHz with no DC link. Efficiency 96.5 % at 3 kW. Still experimental developments.

The soft-switching condition

During the "dead time" t_d the coil current must recharge the C_oss of both switches. The difference from an LLC converter is that in induction heating the load itself changes over time (the workpiece heating, the Curie point). Since 2020, Ambrell, Ajax TOCCO, and ELDEC inverters track the resonant frequency in 100 Hz steps.

Evolution of the device base: IGBT → SiC → GaN

Stage 1: IGBT, up to 100 kHz (1990–2015)

The insulated-gate bipolar transistor. Operating frequencies 50–100 kHz. Modules: Infineon FF400R12KE3, Fuji 2MBI100VA-120-50, Mitsubishi CM450DY-24H. The main limitation is the long "current tail" at turn-off.

Stage 2: SiC MOSFET, 200 kHz – 1 MHz (2015 → present)

Wolfspeed C3M0021120K: 1200 V, 21 mΩ, C_oss ≈ 80 pF, t_on 25 ns, t_off 32 ns. Infineon CoolSiC IMW120R045M1H: 1200 V, 45 mΩ. ROHM SCT4080KE: 1200 V, 80 mΩ.

Stage 3: GaN HEMT, up to 13.56 MHz (2018 → present)

GaN Systems GS66516B: 650 V, 25 mΩ, C_oss ≈ 60 pF, t_on 3.7 ns. EPC GaN EPC2034C: 200 V, 5 mΩ, t_sw 1 ns. Infineon CoolGaN IGT60R070D1: 600 V, 70 mΩ.

In the review by Hatipoğlu and Bodan (M. Hatipoğlu, Ö. Bodan, Applied Sciences 2024, Vol. 14, No. 7996): SiC gives 30 % lower switching losses than IGBT at 100 kHz, and GaN gives a further factor of 2.5 lower than SiC at 500 kHz. At 1 MHz only GaN stays above 90 % efficiency.

Comparison table

Workpiece physics and frequency selection

The system operating frequency is selected for the current penetration depth into the workpiece.

δ_w = √(ρ / (π · μ · f))

Rule of thumb: δ_w = 10–25 % of the workpiece radius for through-heating, or = the case depth for surface hardening. When heated to the Curie point (~770 °C) steel loses its magnetic properties, μ_r drops 200 → 1, and δ_w grows by a factor of 14.

Thermal management and current density

A density of 80–100 A/mm² for water-cooled litz wire is 20–30 times higher than for air cooling. Per unit volume, water removes 4000 times more heat than air.

For a 30 kW coil with 380 W of losses, cooled by water at 8 L/min: ΔT = 0.68 K. Ambrell and EFD design for a ΔT of no more than 5 K at maximum load.

The main problem is the thermal conductivity of the litz-wire insulation: PET 0.15 W/(m·K), PTFE 0.25, polyimide 0.12. A thin 50 µm film over 100 cm² at 380 W gives a 12.7 K drop. For precision coils — thermal pastes or potting compounds.

Water-cooled litz cable: an open line of collaboration

Background

The idea of a water-cooled litz-wire coil was first patented in US 5,461,215 (1995) by the Massachusetts Institute of Technology. In the patent: a bundle of 21,875 strands of AWG #48 (~0.03 mm), 5 sub-bundles in the final twist, 1.5 inch lay, 7.4 mm outer diameter. Deionized water. Tested on a Lepel Corporation induction unit.

It was developed further in US 6,229,126 (2001) and US 6,346,690 (2002). A direct comparison: when transmitting 800 A through a ¼ inch copper tube the losses are ≈ 8 kW, on litz wire ≈ 1.5 kW. A 5.3-fold reduction.

In 2009 the patent CN 201229803Y described a flexible coaxial dual-loop water-cooled cable. In 2023 TE Connectivity patented US 12,476,390 — an induction crimp for litz-wire terminals with water cooling.

Four construction variants

Variant 1. Litz wire inside a hollow PTFE tube, with water washing over the strands directly. Maximum heat removal, but deionized water and up to ~500 V.

Variant 2. Litz wire and a separate water channel in a common PEEK/PTFE jacket. Ordinary tap water, high voltages. Current density 60–80 A/mm².

Variant 3. A coaxial cable with a central water channel (a steel or ceramic tube, with a litz-wire braid over it). YDK Litz Wire (Japan). Minimum parasitic inductance.

Variant 4. A dual-loop coaxial cable (patent CN 201229803Y). Two independent water loops. Maximally flexible, with cooling redundancy.

What the world's manufacturers do

Open problems

• There is no single standard for water-cooled litz cables
• Sealing the ends is a separate engineering task
• The service life of PTFE jackets is 5–7 years, and in practice less
• Monitoring the flow rate and quality of the coolant
• Extreme applications (above 150 °C, aggressive media) — each case is an R&D project

Case 1: a 30 kW induction hardening unit at 250 kHz — what's inside

Let us work through a concrete case — a surface-hardening unit for gears with a module of 4–6 mm. Case depth 1.5 mm.

Input data

• P_out = 30 kW for heating the workpiece
• Three-phase input 380 V / 50 Hz
• Workpiece — 40Kh steel, gear Ø 180 mm
• Case depth 1.5 mm
• Cycle: 8 s heating + 4 s cooling

Frequency calculation

40Kh steel at 800 °C: ρ ≈ 1.2 µΩ·m, μ_r ≈ 1. f = 270 kHz → the standard 250 kHz.

Power chain

Node 1. PFC inductor: Litz PET 705×0.15 mm, 80 turns, Hitachi AMCC-200 core. F_R = 1.18. Losses 90 W.

Node 2. Inverter: A half-bridge on two Wolfspeed CAS300M12BM2. C_res = 5 × KEMET 22 nF/2 kV = 110 nF. Losses 3.5 kW (sw) + 690 W (cond) = 4.2 kW.

Node 3. RF cable: Coaxial, Litz 200 F 630×0.1 mm inner, 630×0.1 mm outer, water cooling channel. Losses 50 W.

Node 4. Matching transformer (1:5): Ferroxcube 3C95, ETD 59/31/22. Primary Litz 200 fPE 255×0.1, 20 turns. Secondary Litz 200 fPE 1260×0.1, 4 turns. P-S-P. F_R = 1.32. Losses 250 + 180 = 430 W.

Node 5. Coil: A single-turn copper tube 8×8 mm. F_R = 4.3, length 0.6 m. Losses 380 W.

Full power balance

P_out (workpiece heating)            = 30 000 W
─────────────────────────────────────────────
System losses:
  PFC inductor                       =    90 W
  Transistors (switching)            = 3 500 W
  Transistors (conduction)           =   690 W
  RF cable                           =    50 W
  Matching transformer               =   430 W
  Coil                               =   380 W
  Capacitors, buses, Graetz bridge   =   860 W
─────────────────────────────────────────────
P_total                              = 36 000 W
System efficiency                    =    83.3 %

83 % efficiency is a normal figure. The best SiC units reach 87–89 %, and up to 91 % with optimizations.

How much litz wire was used

• PFC inductor: 705×0.15 mm, 32 m → 4.2 kg (72 %)
• RF cable: 630×0.1 mm, 8 m → 1.3 kg (23 %)
• Matching transformer: primary 4 m + secondary 1.2 m → 0.3 kg (5 %)
• Coil — a copper tube, no litz wire

Fig. 2. Distribution of litz wire across the nodes of a 30 kW system

Case 2: a 3.5 kW domestic induction cooktop on GaN

A parallel case from an entirely different field.

Input data

• P_out = 3500 W, adjustable 200–3500 W
• Single-phase input 220 V / 50 Hz
• Zone diameter 220 mm
• Monoblock height no more than 70 mm

Frequency calculation

For stainless steel with a ferromagnetic coating at 200–300 °C: ρ ≈ 0.8 µΩ·m, μ_r from 100 to 30. For δ_w = 1 mm: f ≈ 70 kHz. The actual frequency is 80–100 kHz.

Power chain

Here there is no PFC and no matching transformer in the traditional sense. The unit is built on a simplified single-stage topology with a GaN half-bridge directly to the coil.

Node 1. Rectifier + DC link: 4 SiC Schottky diodes, CDE 947D × 6.

Node 2. GaN half-bridge: GaN Systems GS-065-150-1-D2 (650 V, 8.5 mΩ, C_oss ≈ 75 pF). TI LMG1210 drivers.

Node 3. Resonant capacitor: WIMA FKP1 (1000 V, 0.33 µF × 4).

Node 4. Coil — this is where the litz wire sits. A flat spiral of 28 turns, Ø 220 mm. Current 25 A, Cu cross-section 2 mm², the 0.15 mm series, catalog Litz PET 200×0.15 mm. F_R = 1.12. Losses 135 W.

Full power balance

P_out (cookware heating)         = 3 500 W
─────────────────────────────────────────
GaN transistors (switching)      =   55 W
GaN transistors (conduction)     =   65 W
Coil                             =  135 W
Capacitors, rectifier            =   45 W
─────────────────────────────────────────
P_total                          = 3 800 W
System efficiency                =  92.1 %

92 % efficiency for a domestic cooktop is an excellent figure. A typical 2018 IGBT cooktop is 86–88 %, and premium Miele/Bosch units in 2024 are 90–93 %.

How much litz wire

• Coil: 200×0.15 mm, 28 turns of 0.55 m = 15.4 m → 0.38 kg

No PFC and no transformers. That's all of it.

Premium cooktops of 2024

In the premium segment (Miele KMDA 7634, Bosch FlexInduction Pro, AEG TotalFlex) — full-surface coils made of many 60–80 mm coils. Each is litz, the 0.1–0.15 mm series, 80–150 strands. In the Miele Triflex there are up to 22 individual coils, about 1.5 kg of litz wire in total.

Common design mistakes

Mistake 1. Calculating the transformer winding at the fundamental frequency only

With frequency control, in low-power modes the harmonics reach 10–15 %. Litz wire works 3–4 times worse on harmonics. The winding heats up 20–30 °C above the calculated value. Solution: calculate by Sullivan's SFD method (squared field derivative method) through FEMM with the real spectrum. The same methodology as for LLC converters.

Mistake 2. Underestimating the thermal resistance of the insulation

PTFE film loses its yield strength at 150–180 °C. The turns "float," and the tank parameters drift. Solution: PEEK instead of PTFE, or epoxy potting with SiO₂/Al₂O₃.

Mistake 3. Ignoring the resistance of connections at RF

Through a twist or a bolted connection at 250 kHz, 20–40 W is lost with no visible signs. The RF resistance of a contact is 5–10 times higher than DC. Solution: RF crimp lugs (Phoenix Contact NSWS) or tinning in a solder bath using the method from the article on tinning.

Mistake 4. Resonant capacitors with electrolyte

Electrolytics do not work above 100 kHz. At 250 kHz they can dissipate up to 30 % of the power. Solution: only film (KEMET PHE850, WIMA FKP1, Vishay MKP) or ceramic NP0/C0G capacitors.

Mistake 5. Calculating the coil without accounting for coupling to the workpiece

A free coil and a coil with a workpiece have different resistance and inductance. Solution: 3D FEMM with a model of the workpiece. For precision tasks — a multiphysics model with the material properties changing over time.

Mistake 6. Cutting costs on the thermal class

Class A (105 °C) on a coil — the varnish degrades within a month or two. Solution: for power >1 kW use at least class F (155 °C), and for industrial use the 200 °C class.

Fig. 3. Where the current flows at 200 kHz — copper tube vs litz wire

The Russian and global industry

Global leaders

USA: Inductotherm Group (melting up to 5000 kW), Ambrell Corporation (hardening 2–500 kW, the EASYHEAT, EKOHEAT, ECOHEAT series), Inductoheat.

Germany: EFD Induction (1–10000 kW, supplies to BMW, VW, Daimler), eldec Schwenk, HIMMELWERK.

Japan: Fuji Electric, Yashima Densan Co.

China: Inductotherm China, Shenzhen GY, Shuangping Power — mass production up to 100 kW.

Russia: VNIITVCh named after V. P. Vologdin (St. Petersburg, the "Tor," "VChI," "Tiristor" series, 5–1000 kW), JSC RELTEK (Yekaterinburg), Parus-Electro LLC (Moscow), JSC Malien, JSC Zavod Elektrik.

Structure of global litz-wire consumption

• Automotive (hardening, EV chargers, traction motors) — 35–40 %
• Metallurgy and forging — 20–25 %
• Household appliances — 15–20 %
• Welding and brazing — 10 %
• Precision electronics (MEMS, medical, jewelry) — 5–8 %
• R&D — 5–10 %

Trends 2024–2026

A mass shift from IGBT to SiC in the medium and heavy segment. EFD Sinac (2023) — all new models on SiC. Inductoheat StatiPower (2024) — SiC up to 500 kHz. GaN remains below 5 kW, but with a forecast of becoming 25–30 % cheaper by 2027.

For litz-wire manufacturers — a shift to finer series: the 0.2 mm series in 2018 → the 0.1 mm series in 2025 → the 0.071 mm series in 2028.

What we do for induction projects

• 0.2 mm series — domestic and industrial coils at 20–60 kHz
• 0.15 mm series — PFC inductors at 80–150 kHz and low-frequency transformers
• 0.1 mm series — the main one for matching transformers and coils at 150–400 kHz
• 0.071 mm series — RF transformers and coils at 400 kHz – 1 MHz
• 0.05 mm series (to order) — MHz inverters on GaN

Base insulation versions:

• Litz PET — air-cooled PFC inductors, 155 °C, 1000 V
• Litz 200 fPE — the most in-demand for matching transformers and coils, 200 °C, 5000 V
• Litz 200 F — RF cables and high-voltage windings, 200 °C, 3000 V
• Litz 200 PI — precision transformers, polyimide, 200 °C, 3500 V
• LitzA-200 — Arselon aramid fiber up to 240 °C, direct contact with a glowing-hot workpiece

Rectangular forming (the "Pr" index) on any series.

There is no ready-made catalog of "litz wire for induction hardening," just as there is no catalog of "transformer for a server power supply." The same wire by construction (for example, 255×0.1 mm at 200 °C) gives F_R = 1.3 in the primary of a 1:5 transformer, 1.5 in the coil of the same system, and 1.2 in the PFC inductor of a different system. The difference comes from the window geometry, the number of layers, the current regime, and the position relative to the gaps and the workpiece. It makes sense to select litz wire for a specific node and regime, not "in general."

Open collaboration with developers of induction equipment

The DIADA design bureau has been producing continuously transposed litz wire in series since 2020. The core specialization is TU DSGT.685660.019TU (TU 27.32.11-019-59596873-2025).

Line 1. Series production of water-cooled litz cable

The top priority line for 2026–2027. It needs an engineering dialogue with a developer: defining requirements, choosing a construction, R&D on connectors, joint thermal testing. In return — production localization in Russia, individual pricing terms, and priority on lead times.

Line 2. Litz wire for precision applications

Fine series (0.071 and 0.05 mm) for onboard EV charging stations, coils for special-purpose equipment, and laboratory-class RF generators.

Line 3. R&D and pilot batches

Batches from 30 m in any combination of parameters, with a 2–4 week lead time. Each pilot batch comes with a batch certificate. Measuring F_R is a task for the customer's test bench; we provide consulting.

Line 4. Manufacturing cooperation

Long-term contracts with predictable volumes. Reconfiguration to any construction within a few days.

How to start a dialogue

Through the contact form marked "induction heating." It is enough to answer:

1. What induction equipment are you developing?
2. Which frequencies, currents, and power levels are you interested in?
3. Which system nodes require litz wire?
4. Do you need series supply or a one-off batch?

"Induction heating in Russia is growing faster than the expertise in it. We see developers spending months selecting wire by trial and error because there is no one to consult. We want to change that — to be an engineering partner to the industry, not just a supplier." — Igor Toporov, General Director of DIADA

FAQ

Which node of an induction system consumes the most litz wire?

Not the coil. The main consumers are the RF matching transformer (4–8 kg in a 30 kW system) and the PFC inductor (3–5 kg). The coil itself is 1–3 kg, and often just a copper tube with no litz wire at all. A 30 kW system uses 8.5–18 kg of litz wire, about 75 % in the transformer and PFC.

Why can't a copper tube be used in the RF matching transformer?

In a transformer the windings are packed tightly, in multiple layers, surrounded by a core with a strong field. A copper tube would have an F_R of 8–15 at 200 kHz. The transformer turns into a heater of itself. Every manufacturer — Inductotherm, Ambrell, EFD, Inductoheat — builds transformers with litz wire only.

Which induction heating frequency should I choose for my workpiece?

δ_w should be 10–25 % of the radius (through-heating) or equal to the required case depth (surface hardening). For carbon steel: 1–10 kHz for 100–300 mm billets, 20–100 kHz for shafts and gears, 200–500 kHz for gear teeth, 500 kHz – 2 MHz for wire, 2–13.56 MHz for jewelry.

When is a copper tube better than litz wire in the coil?

In three cases: frequencies below 30 kHz at high power; induction melting furnaces next to a 1200 °C melt; the complex geometry of a single-step stamped coil for a non-standard part.

SiC or GaN — which to choose for a new induction inverter?

Below 100 kHz — IGBT. 100 kHz – 1 MHz — SiC MOSFET (Wolfspeed C3M0021120K, Infineon CoolSiC, ROHM SCT4080KE). 1–13.56 MHz — GaN HEMT only (GaN Systems GS66516B). SiC delivers 95–97 % efficiency at 100–500 kHz; GaN is the only device class in off-the-shelf modules above 90 % at 1 MHz.

What is a VITROPERM nanocrystalline core and why is it needed in an RF transformer?

VITROPERM (VACUUMSCHMELZE) and FINEMET (Hitachi/Proterial) are cores made from Fe-Si-B-Cu-Nb ribbon. Their saturation flux density is 1.2 T versus 0.33 T for ferrite — 3–4 times higher. The core is 1.75 times smaller in volume. Critical for EV charging stations and embedded equipment.

Where can I get a water-cooled litz cable in Russia?

A production product with an integrated water channel does not exist in Russia yet. The DIADA design bureau has the full set of process capabilities for all four construction variants, but is launching this as a line for joint development. To start — use the contact form marked "water-cooled induction litz cable."

Can litz wire be used in an induction melting furnace?

Almost never. Melting furnaces run at 1–10 kHz with the winding next to a 1200–1500 °C melt. Litz wire gives no F_R benefit, and thermal radiation destroys polymer insulation within days. The standard is a massive water-cooled copper bus bar.

What is the skin depth in copper at typical induction heating frequencies?

At 100 °C: 0.66 mm at 10 kHz, 0.21 mm at 100 kHz, 0.066 mm at 1 MHz, 0.018 mm at 13.56 MHz. By the formula δ ≈ 2.09 / √f (f in kHz, δ in mm).

What happens to the coil winding when the workpiece passes through the Curie point?

For carbon steel at 770 °C, μ_r drops from 200 to 1 and ρ rises from 0.16 to 1.1 µΩ·m. δ_w increases by a factor of 14. The coupling between coil and workpiece weakens, and the power drops by 30–50 %. The best systems (Ambrell, Inductoheat) retune f_sw in 100 Hz steps.